Recent developments in molecular biology have opened up the possibility of evaluating how social interactions modify gene-expression in the brain, providing opportunities to evaluate how the brain responds to normal as well as pathogenic social interactions. The following work will analyze gene-expression changes using micro-array technology as a function of two distinct types of social dominance (in rodent models) -- those that emerge from emotionally positive friendly playful interactions in juveniles and those based on emotionally negative aggressive interactions in adults. In the first experiment, young rats will be allowed rough-and-tumble play episodes, and sub-regions of the brain from dominant and submissive animals will be evaluated for gene-expression profiles using custom-made gene-chips designed to evaluate approximately 1200 targeted genes representing the dynamics of major neurochemical systems of the brain. Using comparable behavioral procedures, adult rats will be allowed to participate in agonistic encounters in a resident-intruder paradigm, and expression profiles of the same genes will be evaluated and contrasted. Control studies will also be conducted to determine the role of prior-social isolation in these gene-changes, as well as those arising from benign social contact that entails no vigorous social interactions. This work provides an empirical interface between modern molecular biological approaches to brain functions and several robust neuro-ethological strategies for studying basic social processes in animal models. Also, if we consider that loss in social encounters is one of the major factors in the etiology of depression, the work has the potential to shed light on sociopathological processes that can initiate the cascade of brain events toward depression, and thereby provide new ideas for biological interventions that may retard the depressive cascade